1. Field of the Invention
This invention relates to a method of evaluating the amount of precipitated oxygen in a silicon wafer.
2. Description of the Prior Art
A semiconductor integrated circuit (IC) is manufactured by following process; on a mirror-polished silicon wafer (hereinafter referred to as silicon wafer), which is made of single crystal of silicon grown by the Czochralski method, the floating zone method, and etc. and has a thickness of about 0.5 mm, integrated circuit patterns such as diodes, transistors, MOS-FETs, resistors, capacitors and so on are formed, then the surface of the silicon wafer is covered with a thin insulating film followed by forming metal contacts and interconnections.
For the IC manufacture, polycrystalline silicon, for instance, is melted in a quartz crucible, and a single crystal rod which serves as seed crystal is brought from above the crucible into contact with the silicon melt while slowly rotating the rod. Silicon single crystal is thus grown by the Czochralski method. However, the single crystal thus grown inevitably contains about 10.sup.18 /cm.sup.3 of over-saturated interstitial oxygen. When the silicon wafer which contains interstitial oxygen is subsequently annealed at a low temperature of 800.degree. C. or below and also at a high temperature of 1,000.degree. C. or above, for instance, the interstitial oxygen is readily precipitated as oxides of silicon, thus generating numerous micro defects in the single crystal.
The micro defects suitably constitute getting cites of heavy metal impurities, etc., so long as they are found in the bulk of the wafer. However, when they exist in the vicinity of the wafer surfaces, they have direct adverse effects on the yield of device and so forth.
Therefore, the evaluation of the amount of precipitated oxygen in silicon single crystal is becoming increasingly important.
Heretofore, in case of silicon single crystal grown by the Czochralski method, the evaluation of the amount of precipitated oxygen after the single crystal has been heat treated, is made by using a Fourier transform type infrared absorption (FT-IR) method.
This method makes use of the absorption of light of a particular wavelength in the infrared wavelength range by interstitial oxygen atoms present in the silicon single crystal. In this method, the absorption spectrum in the infrared wavelength range is first determined, then the interstitial oxygen concentration is determined from the peak height of the spectrum, and then the amount .DELTA.Oi of the precipitated oxygen is determined from the difference between the interstitial oxygen concentrations Oi(b) and Oi(a) in silicon wafer before and after a heat treatment thereon, respectively.
According to the prior art as described above, the amount of precipitated oxygen .DELTA.Oi is determined from the difference between the interstitial oxygen concentrations Oi(b) and Oi(a) before and after the heat treatment. Therefore, with a sample which has been heat treated and in which the initial interstitial oxygen concentration is not known, it is impossible to determine the amount of precipitated oxygen.
In another aspect of the above prior art, as transmission process is adopted, in which infrared light transmitted through the silicon single crystal are detected. Therefore, although the amount of precipitated oxygen can be evaluated in the entire thickness of the sample, in a superficial region thereof to a certain depth non-destructive evaluation can not be made without making the sample thinner.
Further, in the case of low resistivity single crystal silicon which is doped with a high concentration of a dopant for resistivity control, infrared lights are absorbed pronouncedly by free carriers, so that they are difficult to be transmitted. Therefore, it is necessary to make the sample extremely thin, but this is unfeasible.